Two-stage drawing process for polybenzimidazole fiber

ABSTRACT

POLYBENZIMIDAZOLE FIBER IS DRAWN IN TWO-STAGES WHERE IN THE SECOND DRAW IS AT A LOWER TEMPERATURE THAN THE FIRST. AFTER DOUBLE DRAWING THE TENACITY AND INITIAL MODULUS OF THE FIBER ARE HIGHER THAN ATER THE FIRST STAGE DRAW. THE FIRST STAE DRAW MAY BE EFFECED AT A TEMPERATURE IN THE RANGE 300-650*C. WITH A RATIO OF 1.5-3.5X WHILE THE SECOND STAE DRAW MAY E CONDUCTED AT A LOWER TEMPERATURE IN THE RANGE 200-500*C. WITH A RATIO OF 1.05-1.5X. VARIOUS MEANS OF HEATING ARE DISCLOSED, INCLUDING RADIATION AND CONTACT WITH HOT METAL SURFACES. EXPSURE TIME IS DISCLOSED TO BE FUNCTION OF THE MODE OF HEATING AND THE TEMPERATURE USED.

United States Patent U.S. Cl. 264-290 R 11 Claims ABSTRACT OF THE DISCLOSURE Polybenzimidazole fiber is drawn in two-stages wherein the second draw is at a lower temperature than the first. After double drawing the tenacity and initial modulus of the fiber are higher than after the first stage draw. The first stage draw may be efiected at a temperature in the range 300650 C. with a ratio of 1.5-3.5x while the second stage draw may be conducted at a lower temperature in the range 200-500 C. with a ratio of 1.05-l.5 X. Various means of heating are disclosed, including radiation and contact with hot metal surfaces. Exposure time is disclosed to be a function of the mode of heating and the temperature used.

CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part of our co-pending U.S. Ser. No. 770,083, filed Oct. 23, 1968.

' BACKGROUND OF THE INVENTION Double drawing, that is, two stage drawing, of polymer fibers or filaments is a well known method for increasing tensile properties, particularly tenacity. Heretofore, however, the second draw has been conducted at a higher temperature than that employed in the first draw, especially when working with high molecular weight polymers such as nylons, polyesters, and polyolefins. See, for instance, U.S. Pat. Nos. 2,556,295 to Pace and 3,400,194 to Boone et al. which disclose two stage polyester hot drawing processes, the second stage of which is conducted at more severe conditions than the first stage. It was thought that these high molecular weight polymer fibers needed a more severe second draw treatment because of their increased stiffness after the first draw.

It is an object of the present invention to provide a process for improving the tensile properties of polybenzimidazole fiber.

It is another object of the invention to provide an improved two stage hot drawing process for polybenzimidazole fiber.

These and other objects, as well as the scope, nature and utilization of the process will be apparent from the following detailed description and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for improving the tensile properties of polybenzimidazole fiber comprises subjecting the fiber to a first draw treatment at a temperature in the range of from 300 to 650 C. and at a draw ratio in the range of from 1.5 :1 to 3.5 :1, and thereafter subjecting the resulting drawn fiber to a second draw treatment at a draw ratio in the range of from 1.05:1 to 1.5 :1 and at a temperature which is lower than that used in the first draw and which is within the range of from 200 to 500 C. to produce a polybenzimidazole fiber product having a higher tenacity and higher initial modulus than that of the fiber product obtained from the first draw treatment.

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DESCRIPTION OF PREFERRED EMBODIMENTS The starting polymer Polybenzimidazoles are a known class of heterocyclic polymers, which consist essentially of recurring units of the following Formulas I and II.

Formula I is:

wherein R is a tetravalent aromatic nucleus, with the nitrogen atoms forming the benzimidazole rings being paired upon adjacent carbon atoms, i.e., ortho carbon atoms, of the aromatic nucleus, and R is a member of the class consisting of an aliphatic (alkylene) group, a cycloaliphatic ring, an aromatic ring and a heterocyclic ring such as pyridine, pyrazine, furan, quinoline, thiophene, and pyran.

Formula H is:

N C% \Z; l

wherein Z is an aromatic nucleus having the nitrogen atoms forming the benzimidazole ring paired upon adjacent carbon atoms of the aromatic nucleus.

Preferably, aromatic polybenzimidazole fibers are used in the present invention, that is, fibers prepared from polymers consisting essentially of the recurring units of Formula II and Formula I wherein R is an aromatic ring or a heterocyclic ring.

As set forth in US. Pat. No. 3,174,947 and U.S. Reissue Pat. No. 26,065, which are incorporated herein by reference, the aromatic polybenzimidazoles having the recurring units of Formula H may be prepared by selfcondensing a trifunctional aromatic compound containing only a single set of ortho disposed diamino substituents and an aromatic, preferably phenyl, carboxylate ester substituent. Exemplary of polymers of this type is poly- 2,5(6)-benzimidazole prepared by the auto-condensation of pheny1-3,4-diaminobenzoate.

As also set forth in the above-mentioned patents, the aromatic polybenzimidazoles having the recurring units of Formula I may be prepared by condensing an aromatic tetraamine compound containing a pair of ortho-diamino substituents on the aromatic nucleus with a dicarboxyl compound selected from the class consisting of (a) the diphenyl ester of an aromatic dicarboxylic acid, (b) the diphenyl ester of a heterocyclic dicarboxylic acid wherein the carboxyl groups are substituents upon carbon in a ring compound selected from the class consisting of pyridine, pyrazine, furan, quinoline, thiophene and pyran and (c) an anhydride of an aromatic dicarboxylic acid.

Examples of aromatic polybenzimidazoles which have the recurring structure of Formula I and which may be formed into fibers and subjected to the double drawing process of the present invention include:

poly-2',2' (m-phenylene)-5,5'-di(benzimidazole) methane;

poly-2,2" (m-phenylene)-',5"-di(benzimidazole) propane-2,2; and

poly-2,2" (m-phenylene)-5,5" di(benzimidazole) ethylene-1,2

where the double bonds of the ethylene groups are intact in the final polymer.

The preferred aromatic polybenzimidazole fiber is one prepared from poly-2,2-(m-phenylene)-5,5'-bibenzimidazole, the recurring unit of which is:

Any polymerization process known to those skilled in the art may be employed to prepare the polybenzimidazole which may then be spun into fiber and subjected to the double drawing process of the present invention. With respect to aromatic polybenzimidazoles, preferably, equimolar quantities of the monomeric tetraamine and dicarboxyl compound are introduced into a first stage melt polymerization reaction zone and heated therein at a temperature above about 200 C., preferably at least 250 C., and more preferably from about 270 to 300 C. The reaction is conducted in a substantially oxygen-free atmosphere, i.e., below about 20 p.p.m. oxygen and preferably below about 8 p.p.m. oxygen, until a foamed prepolymer is formed. Usually, the first stage reaction is continued until a prepolymer is formed having an inherent viscostiy, expressed as deciliters per gram, of at least 0.1, and preferably from about 0.13 to 0.3, the inherent viscostiy (I.V.) as used in the present specification and claims being determined from a solution of 0.4 gram of the polymer in 100 ml. of 97 percent H 50 at 25 C.

After the conclusion of the first stage reaction, which normally takes at least 0.5 hour and preferably 1 to 3 hours, the foamed prepolymer is cooled andthen powdered or pulverized in any convenient manner, The 'resulting prepolymer powder is then introduced into a second stage polymerization reaction zone wherein it is heated under substantially oxygen-free conditions, as described above, to yield a polybenzimidazole polymer prod uct, desirably having an I.V., as measured above, of at least 0.6 e.g., 0.80 to 1.1 or more.

The temperature employed in the second stage is at least 250 C., preferably at least 325 C., and more preferably from about 350 to 425 C. The second stage re action generally takes at least 0.5 hour, and preferably from about 1 to 4 hours or more.

Preparation of the fiber As is well known, the polybenzimidazoles are generally formed into fibers or filaments by dry or wet spinning a solution of the polymer in an appropriate solvent such as dimethylacetamide, dimethylformamide, dimethylsulfoxide or sulfuric acid through an opening of predetermined shape into an evaporative atmosphere for the solvent in which most of it is evaporated, or into a coagulation bath, resulting in the polymer having the desired shape.

The polymer solutions may be prepared, forexample, by dissolving sufiicient polybenzimidazole in the solvent to yield a final solution suitable for extrusion containing from about to 45 percent by weight of polymer, based on the total weight of the solution, preferably from about to 30 percent by weight.

One suitable means for dissolving the polymer in the solvent is by mixing the materials at a temperature above the atmospheric boiling point of the solvent, for example, to 120 C. above such boiling point, and at a pressure of 2 to 15 atmospheres for a period of 1 to 5 hours.

Preferably, the above polymer solutions, after suitable filtration to remove any undissolved portions, are dry spun. For example, the solutions may be extruded through a spinneret into a conventional type downdraft spinning column containing a circulating inert gas such as nitrogen, noble gases, combustion gases or super heated steam. Conveniently, the spinneret face is at a temperature of from to 170 C., the top of the column from about 120 to 220 C., the middle of the column from about to 250 C., and the bottom of the column from about 60 to 320 C. After leaving the spinning column, the fibers are taken up, at a speed in the range of from about 50 to 350 meters per minute, washed thoroughly such as with water in order to remove residual solvent, and then dried in any suitable type of apparatus at a temperature in the range of from about to 300 C. for about 2 to 100 hours or more.

Prior to being washed the dry-spun fibers are subjected to a slight steam drawing treatment at a draw ratio of from about 1.1:1 to 15:1.

This pre-wash drawing treatment is to be distinguished from the post-wash double drawing process of the present invention.

The present invention is directed to the post-wash drawing of the polybenzimidazole fibers and not their initial formation; therefore, any of the known methods for preparing the fibers in addition to those described above may be used.

First and second drawing stages The resulting washed and dried polybenzimidazole fiber, preferably aromatic polybenzimidazole fiber, is then drawn (first draw) at a temperature in the range of about 300 to 650 C. at a draw ratio in the range of 1.5:1 to 35:1, and thereafter is drawn (second draw) at a temperature lower than that used in the first draw which is in the range of from about 200 to 500 C. at a draw ratio of about 1.05:1 to 1.5:1 to produce a polybenzimidazole fiber product having a higher tenacity and higher initial mod ulus than that of the fiber product obtained from the first draw treatment.

The first draw treatment is preferably conducted at a temperature in the range of 450 to 600 C., and the second draw treatment is preferably conducted at a temperature in the range of 390 to 470 C. which is below that used in the first draw treatment.

The first draw treatment is preferably conducted at a draw ratio of 2.0:1 to 32:1, and the second draw treatment is preferably conducted at a draw rtaio of about 1.1:1 to 1.3:1.

The manner in which heat is applied to the polybenzimidazole fiber undergoing hot drawing may be varied widely. For instance, the fiber may be heated viaradiation heating by passage through a muffie furnace or other hot gas heating zone. Alternatively, the polybenzimidazole fiber undergoing hot drawing may be heated via conductive heating wherein the fiber is passed over a hot surface, such as one or more hot shoe, roll, plate, pin, etc.

As will be apparent to those skilled in the art, the residence time during which the fiber is being drawn at the elevated temperature indicated varies with the mode of heating selected. For instance, a generally longer residence time is commonly utilized when the mode of heating is via radiation, than when the mode of heating is via conduction.

When the mode of heating is via radiation, the first draw treatment residence times commonly range from about 5 to 30 seconds (preferably 10 to 20 seconds), and the second draw treatment residence times commonly range from about 1 to 20 seconds (preferably 3 to 15 seconds). When the mode of heating is via conduction, the first draw treatment residence times commonly range from about 0.2 to 3 seconds (preferably 0.3 to 1 second), and second raw treatment residence times commonly range from about 0.1 to 1.5 seconds (preferably 0.2 to 0.8 second). The exact residence time selected for optimum results will also be influenced by the temperature of the draw treatment as will be apparent to those skilled in the art.. v V

As previously mentioned, the essence of the present invention is the discovery that the second draw treatment can be advantageously conducted at milder conditions and not more strenuous or severe conditions as would be expected. Not only can the second draw be treatment (post-wash draw) by drawing it in a mufile furnace maintained at a temperature of 550 C. The draw ratio used was 22:1 and the residence time of the yarn in the furnace was 12 seconds.

. This intermediate drawn fiber was then used as the feed fiber to a second rawing stage in the muflle furnace, wherein the temperature and draw ratio were varied as shown in Table I below.

TABLE I Resi- Supply Tempera- Elonga- Initial dence roll rate, ture in Percent Denier tion, Tenacity, modulus. time in Draw meters] degrees of draw per pergrams] grams] Experlment Number seconds ratio 1 minute centigrade break 1 fiber cent 3 denier Try 4 denier I. Feed fiber from first stage draw II. Second stage draw conditions and results 7 1. 21 55 560 40 3. 5 29 2. 9 66 7 1.32 a 55 560 60 3. 1 25 3.4 17 75 7 V 1. 42 55 560 80v 2. 8 19 2. 8 12 74 7 1. 41 55 530 40 3. 0 16 3. 3 13 80 7 1. 61 55 530 60 2. 7 14 3. 7 14 88 7 1. 71 55 500 60 2. 6 9 4. 5 14 127 7 1. 93 55 500 80 2. 1 6 4. 9 12 141 7 1. 16 55 470 40 3.3 17 5.9 24 126 7 1. 25 55 470 60 3. 2 12 5. 7 20 134 7 1. 14 55 440 40 3. 7 16 5. 7 23 130 7 1. 19 55 440 55 3. 4 12 6. 6 23 156 7 1. 24 55 440 70 3. 3 9 6. 9 21 159 7 1. 09 55 425 25 3. 8 23 5. 4 26 114 7 l. 14 i 55 425 40 3.3 13. 2 5. 9 22 140 7 1.19 55 425 55 3. 2 10 6. 4 21 146 7 1. 24 55 425 70 3. 1 8 7. 1 20 162 V teed 1 Tile draw ratio is the rate at which the stretch roll or take-up roll is rotated divided by the rate of rotation of the supply or 2 Draw break is the draw ratio at which the yarn breaks.

8 Measured at the breakpoint of the yarn.

4 TE}/ =Index oi fiber organization wherein T is tenacity at break in grams per denier and E is elongation of percent extension from original length at break in tenslle test. An explanation of this test and its significance is given in the Textile Research 7 Journal 36, No. 7, pages 593-602, July 1966 5 Stress of tangent of initial slope at 2% elongation extended to 100% elongation. Is measure of stifiness in the elastic region and in the region of use.

conducted at a lower temperature within the range of 200 to 500 0., but a lower draw ratio and residence time may also be employed to produce a polybenzimidazole fiber product having a higher tenacity and initial modulus values than the intermediate product fiber obtained fromthe first draw. The use of the milder or less severe conditions in the second draw treatment to obtain the improved, tensile properties is completely contrary to known high molecular weight polymer fiber drawing technology with respect to two stage drawing. The drawing operation (first and second drawing treatmerits or stages) may be conducted on a batch, contiiiuous or semi-continuous basis, and preferably is conducted on a continuous basis. 1

The present invention is equally applicable to the drawing of monoor multi-filaments or fibers, for example, yarns having 10 to 1000 filaments or more. The denier of the fiber used in the present invention is not critical," commonly being from about 1 to 20 denier per fiber (D.P.F.).

The following examples are given as specific illustrations of the unexpected results achieved when practicing the process of the present invention.

EXAMPLE 1 Poly-2,2'-(m-phenylene) 5,5 bibenzimidazole (prepared by the polymerization of 3,3'-diaminobenzidine and diphenyl isophthalate) was dissolved in'dime'thyl acetamide. The resulting polymer solution (23 percent by weight solids) was extruded through a hole spinneret into a downdraft spinning column containing circulating nitrogen to produce a 50 filament yarn of 400 total denier.

After leaving the spinning column, the yarn was taken As can be seen from the data of Table I, the less severe conditions used in the second stage draw treatment yield a polybenzimidazole yarn of increased tensile properties (i.e. tenacity and initial modulus). In Experiment Nos. 1, 2, and 3 the temperature of the second stage draw treatment (i.e. 560 C.) was in excess of that employed during the first stage draw treatment, and resulted in a decrease in both the tenacity and initial modulus values. In Expermient Nos. 4 and 5 the temperature of the second stage draw treatment (i.e. 530 C.) was less than that employed during the first stage draw treatment, but was still excessive. In Experiment Nos. 5, 6, and 7 the second stage draw ratio was excessive, and the tenacity was decreased. Accordingly, Experiment Nos. 1 through 7 are presented for comparative purposes, and Experiment Nos. 8 through 16 are in accordance with applicants improved process.

Similar results are obtained when a mono-filament polybenzimidazole is used in place of the yarn, as well as when other polybenzimidazole fibers or yarns such as those specifically mentioned heretofore are used.

EXAMPLE II A washed and dried polybenzimidazole yarn [poly-2,2- (m-phenylene)-5,5'-bibenzimidazole] substantially similar to that subjected to the drawing treatments of Example I was drawn via conductive heating rather than radiation heating. The yarn consisted of 50 continuous filaments and had a total denier of about 600. The yarn exhibited a single filament tenacity of about 1.7 grams per denier, an elongation of about percent, and an initial modulus of about 38 grams per denier.

The yarn was subjected to a first drawing treatment (post-wash draw) by drawing while in moving contact with a pair of convex heated drawing shoes placed in an end-to-end relationship. Each drawing shoe had a length of 10 inches and a width of 1%. inch, was resistance heated to the temperature indicated in Table II, and was provided with a smooth stainless steel alloy surface. During the first drawing treatment the drawing shoes were maintained at 430 C. The draw ratio was 20:1, and the residence time of the yarn while in contact with the drawing shoes was about 1 second.

This intermediate drawn fiber was then used as the feed fiber to a second drawing stage on a pair of like drawing shoes at draw ratio of 1.2:l and a contact time of about 0.6 second at various temperatures as shown in Table H.

8 improved process the benefits of the present process are clearly apparent since both the final tenacity and the initial modulus values exceed those obtained after the first draw stage. 1

EXAMPLE IV Example H was repeated with the modifications indicated. The draw ratio during the first draw treatment TABLE II Resi- Supply Tempera- Initial dence roll rate, ture in Denier Elonga- Tenacity, modulus, Experiment tlme 1n Draw meters] degrees per tion, grams/ grams] Number seconds ratio minute eentigrade fiber percent denier TE 1/2 denier I. Feed fiber from first draw II. Second stage draw conditions and results As can be seen from the data of Table II, the less severe conditions used in the second stage draw treatment yield a polybenzimidazole fiber of generally tensile properties (i.e. tenacity and initial modulus). In Experiment Nos. 1 through 3 the temperature of the second stage draw treatment was excessive, and resulted in a was 3. 0:1, and the residence time while in contact with the first stage drawing shoes was about 0.7 second.

This intermediate drawn fiber was then used as the feed fiber to a second drawing stage on a pair of like drawing shoes at a draw ratio of 1.2:1 and a contact time of about 0.3 second as shown in Table IV.

TABLE IV Resi- Supply Temperalnitial deuce roll rate, ture in Denier Elonga- Tenacity, modulus Experiment time in Draw meters, degrees per tion, 1 grams} grams Number seconds ratio minute centigrade fiber percent. denier TE 112 denier I. Feed fiber from first draw II. Second stage draw conditions andresults 0. 3 1. 2 75 445 3. 0 22. 1 4'. s 22. 7 a2 0.3 1.2 76 430 2. 9 13.8 4.7 17. 3 02 0. 3 1. 2. 75 415 2. 8 12.2 5. 3 l8. 5 110 0. 3 1. 2 75 400 3.0 8. 7 5. 6 16. 4 119 decrease in both the tenacity and initial modulus values. In Experiment No. 4 the tenacity was decreased slightly while the initial modulus was enhanced. Experiment No. 5 indicates that both the tenacity and the initial modulus were enhanced, and is in accordance with applicants" improved process.

EXAMPLE III Example II was repeated with the modifications indicated. The draw ratio during the first draw stage was 3.011, and the residence time while in contact with the first stage drawing shoes was about 0.7 second.

This intermediate drawn fiber was then used as the feed fiber to a second drawing stage on a pair of like drawing shoes at a draw ratio of 1.1:1 and a contact time As can be seen'from the data-of Table IV, the less severe conditions u'sed in the second stage draw treatment yield a polybenzimidazole fiber of increased tensile properties (i.e. tenacity and initial 'modulus), In Experiment Nos. 1 and 2 the temperature of the second stage draw treatment was excessive, andresulted in a decrease inthe tenacity and initial modulus values. Experiment Nos. 3 and 4 indicatethat both the tenacity and the initial modulus were enhanced in accordance with applicants improved process.

As will be apparent to those skilled in the art, variations and modifications may be resorted to which depart from the specific conditions employed in the examples.

of about .4 second as shown in Table III. Such vanatlons and modifications are to be considered TABLE III Resi- Supply Tempera- Initial deuce roll rate, ture in Denier Elonga- Tenacity, modulus, Experiment time in Draw meters/ degrees per tion, grams/ grams] Number seconds ratio minute centigrade fiber percent denier TE l/2 denier I. Feed fiber from first draw II. Second stage draw conditions and results 0.4 1.1 445 3.4 24. 4.5 a 22.3 76 I 0. 4 1. 1 75 430 3. 4 ,22. 7 5. 1 24. 2 s4 0. 4 1.1 75 415 3. 5 22. 0 5. 0 23. 6' 81 0. 4 1. 1 75 400 3.3 15. 4 5. 7 22.2 102 As can be seen from the data of Table III, the less within the scope and purview of the claims appended severe conditions used in the second stage draw treat- 9 hereto. ment yield a polybenzimidazole fiber of generally 1n- We claim: I

creased tensile properties (i.e. tenacity and initial modulus). In Experiment Nos. 1 and 2 the temperature of the second draw treatment was excessive. In Experiment No. 4 which was conducted in accordance with applicants l. A process for improving the tensile properties of polybenzimida'zole fiber, which comprises subjecting said fiber to a first draw treatment ata temperature in the range of from 300 to 650 C and at a draw ratio in the range of from 1.5 :1 to 3.5 :1, and thereafter subjecting the resulting drawn fiber to a second draw treatment at a draw ratio in the range of from 1.05:1 to 1.5:1 and at a temperature which is lower than that used in the first draw and which is withinthe range of from 200 to 500 C. to produce a polybenzimidazole fiber product having a higher tenacity and higher initial modulus than that of the fiber product obtained from said first draw treatment.

2. A process according to claim 1 wherein said polybenzimidazole fiber is an aromatic polybenzimidazole fiber.

3. A process according to claim 1 wherein said polybenzimidazole is poly-2,2-(m-phenylene)-5,5'-bibenzimidazole.

4. A process according to claim 1 wherein said first draw is conducted via radiation heating for a period of time in the range of from {to 30 seconds at a draw ratio of from 2.0:1 to 32:1, and said second draw treatment is conducted via radiation heating for a period of time in the range of from 1 to 20 seconds at a draw ratio of from 1.1:1 to 13:1."

5. A process according to claim 1 wherein said first draw is conducted via conduction heating for a period of time in the range of from 0.2 to 3 seconds, and said second draw treatment is conducted via conduction heating for a period of time in the range of from 0.1 to 1.5 seconds at a draw ratio of from 1.1:1 to 1.321.

6. A process according to claim 2 wherein said resulting draw fiber from said first draw treatment is subjected to a second draw treatment ata lower temperature, residence time and draw ratio than employed in said first draw treatment.

7. A process for improvingthe tensile properties of polybenzimidazole fiber, which comprises subjecting said fiber to a first draw treatment via radiation heating at a temperature in the range of from 450 to 600 C. for a period of time of from 5 to 30 seconds at a draw ratio of from 1.5:1 to 3:521, and thereafter subjecting the resulting drawn fiber to a second draw treatment via radiation heating at a lower temperature than the first draw which is within the range of from 390 to 470 C. for a period of from 1 to 20 seconds at a draw ratio of from 1.05:1 to 1.5 :1 to produce a polybenzimidazole fiber product having a higher tenacity and higher initial modulus than that of the fiber product obtained from said first draw treatment.

8. A process according to claim 7 wherein said polybgnzimidazole fiber is an aromatic polybenzimidazole fi er.

9. A process according to claim 7 wherein said polybenzimidazole is poly-2,2-(m-phenylene)-5,5'-bibenzimidazole.

10. A process according to claim 7 wherein said first stage draw treatment is'conducted at a draw ratio of from 2.0:1 to 3.2:1, and said second stage draw treatment is conducted at a draw ratio of from 1.1:1 to 13:1.

11. A process according to claim 10 wherein said resulting drawn fiber from said first draw treatment is subject to a second draw treatment at a lower temperature, residence time and draw ratio than employed in said first draw treatment.

References Cited UNITED STATES PATENTS 3,400,192 9/1968 Hartmann 264-290 T 3,441,640 4/1969 S antangelo 264-203 3,526,693 9/1970 Rnlison et a1. 264-290 R 3,541,199 11/1970 Bohrer et al 264-290 R 3,584,104 6/1971 Bohrer et al 264-290 R HERBERT MINTZ, Assistant Examiner US. Cl. X.R. 264-204 

